In robotic welding applications, a welding gun is typically mounted at the free end of the robot. A relatively heavy, mechanical, break-away clutch has proved undesirable, especially with heavier spot welding guns. In these situations, it is desirable to employ a small, relatively light-weight, electronic tactile sensor. The tactile sensor and its associated circuitry must be able to sense an overforce condition such as the robot being stuck to the workpiece or colliding with a fixture.
Sensors for use on robots identify the position and/or orientation of an object; measure the force between the manipulator and an object, or the torque on the manipulator; identify and distinguish one object from several others; determine the position of the robot; and inspect the quality of an object.
Non-contact sensors include visual systems and also include the use of microwaves, liquid jets and photoelectric beams. Contact sensors generally include the use of slip sensors, strain gauges or probes.
Numerous patents are directed to sensing force such as between the robot arm and the object being manipulated or the torque on the manipulator. U.S. Pat. No. 3,904,234 discloses a mechanism for measuring the magnitude of force exerted by a gripper on an object and indicating whether the object is slipping out of the gripper. The force of the grip is measured using proportional sensors or transducers.
Other patents directed to the sensing of force and torque in a robotic environment include U.S. Pat. Nos. 3,952,880; 3,921,445; 4,132,318; and 4,320,392.
Numerous patents show systems which have force-limiting features including circuits for providing alarm and stop signals. For example, U.S. Pat. No. 3,948,093 to Folchi et al discloses a force transducer wherein the moment at a basic module can be measured by the voltage output of a bridge associated with strain gauges.
Another example is shown by U.S. Pat. No. 4,369,663 to Venturello et al. The Venturello et al patent discloses a transducer having detectors which are arranged to measure the capacitance of a number of capacitors and thereafter provide an electrical signal indicative of the forces and moments applied to a movable member.
U.S. Pat. No. 3,919,563 to Lautier et al discloses a safety fastener which automatically releases upon sensing and transducing at least one force which is applied to a fastening element of the fastener. Means are provided for integrating a signal from the sensing and transducer means and comparing the signal with a given least reset value for the integrating means. Means are also provided for comparing the signal integral to a given maximum value. The fastener is unlocked when the integral is above the maximum value.
One problem that should be addressed by any such signal processing circuit is when two forces appear along two different axes of the tactile sensor. The forces should not cancel themselves and, thereby, fail to deliver a stop signal.